This invention relates to a method of treating sulfur oxide and sulfate absorbent pellets wherein calcium compounds are the active absorbing agents to improve the physical characteristics of the pellets. More specifically, the subject invention relates to a method of treating pellets containing calcium oxide and calcium hydroxide so that they are dimensionally stable when employed to absorb sulfur oxides or sulfuric acid from high temperature combustion gases of sulfur containing hydrocarbon fuels.
It is well known that solid calcium hydroxide reacts with gaseous sulfur trioxide and sulfuric acid at elevated temperatures to form solid calcium sulfate. Thus, calcium hydroxide has been investigated as an absorber to entrain sulfates or sulfur oxides from hot combustion exhaust streams. Porous, unsintered pellets of calcium hydroxide containing small amounts of sodium oxide and silicon dioxide as binders have been formed for this purpose. Porous pellets are deemed a convenient form for the absorbent because they provide a large absorbent surface area and reproducible quantities can readily be loaded into fixed volume scrubbing vessels to treat exhaust gases without the pellets being carried away. However, when confined and closely packed in such a vessel and then exposed to a hot exhaust stream containing sulfur oxides, the pellets expanded and crumbled. This resulted in a significant flow restriction on the system. It is thus apparent that although calcium hydroxide based pellets are desirable for use in exhaust scrubbing applications because they are good sulfate absorbers, inexpensive to produce and nontoxic both before and after sulfate absorption, they cannot be used because of their poor physical characteristics. See the final report of phase one, Contract No. 68-03-0497 between Exxon Research and Engineering Company, Linden, N.J., and the Environmental Protection Agency, Ann Arbor, MI ( 1975).
I have discovered a method of treating calcium compound containing pellets so that they absorb gaseous sulfates without physical degradation. It is an object of my invention to provide a method of treating pellets initially containing calcium oxide and calcium hydroxide to transform at least a portion thereof to calcium carbonate. In contrast to the prior art pellets, pellets treated by my method are effective and durable in fixed volume sulfate scrubbers.
In accordance with a preferred embodiment of my invention, the above and other objects are accomplished by making calcined pellets nominally containing 85% by weight calcium hydroxide including traces of calcium oxide and carbonate, and about 15% of a sodium silicate based binder wherein the ratio of silica to sodium oxide is about two to one. The pellets are heated in an open container with carbon dioxide gas. For example, a 10% carbon dioxide-90% nitrogen atmosphere is suitable. The reaction with carbon dioxide is carried out at a temperature of at least 200.degree. C. but no greater than 600.degree. C. until a substantial percentage of the calcium has been converted to CaCO.sub.3. At lower temperatures the conversion is too slow and at higher temperatures the strength of the pellets is decreased. The treated pellets are suitable for prolonged and effective use in fixed volume sulfate scrubbers.
In the preferred practice of my invention, pellets are formed from a major portion of a calcium compound convertible to calcium hydroxide and calcium oxide with calcining, and a minor portion of a sodium silicate binder. A higher proportion of the calcium compound ultimately increases the in-scrubber sulfate absorbing capacity of the pellets. However, at least 10% by weight of the sodium silicate binder is desirable in the calcined pellets to bind them together. Attempts to form pellets containing calcium carbonate in a single step without first forming pellets containing calcium oxide and calcium hydroxide and then treating them with carbon dioxide to form the carbonate were unsuccessful.